Efficiency audible alarm

ABSTRACT

An audible alarm has first and second acoustic chambers that deliver sound to respective first and second horns. A phase adjustment circuit for delays the phase of sound generated in one of the two chambers so that sound emerging from the two horns is delivered with the same phase and same frequency, and emerging sound from the two horns is additive.

RELATED APPLICATION

This application claims priority to Provisional Application Ser. No.60/______, filed December, 2002 entitled Improved Efficiency AudibleAlarm.

BACKGROUND

The present invention relates to the field of audible alarm devices ingeneral, and in particular to that class of audible alarms that utilizesa driven vibrating member in conjunction with a resonance chamber toproduce a loud sound roughly at a system resonant frequency and/or itsmultiples. The disclosed invention provides a general method forsignificantly improving the efficiency of sound production of suchaudible alarms, and it demonstrates several means for achieving suchenergy efficiency through example devices whose shape, structure andconstruction enable the realization of the method.

The audible alarm is one of the most ubiquitous of all devices, and itsmanifestations range widely from the “clang” of a church bell to the“shriek” sound of a siren to the “click” sound of a tactile keyboardswitch. It is a fundamental objective of all audible alarms to providethe loudest, most recognizable sound possible; it is also desirable toproduce such sound with the lowest possible expenditure of energy, i.e.to function efficiently.

One very large class of audible alarms, herein referred to as a “plateand chamber” alarm, is characterized by a mechanical, vibration “plate”which works in companion with an acoustic resonance “chamber”. Thesealarms commonly find application in such familiar devices as smokedetectors and/or carbon monoxide detectors, open door enunciators,vehicle backup warning devices, and automotive horns. Such alarms oftenuse battery power as their primary or backup source of power, and theamount of available energy stored in the battery can be a limitingfactor for the overall performance of the alarm. In battery-poweredalarms in particular, it is highly desirable to convert the input poweravailable to the alarm into the maximum amount of acoustic output power,i.e., for the alarm to be as energy efficient as possible. Thus it ishighly desirable to obtain increased acoustic efficiency, either as anincrease in loudness, a decrease in power, or a combination of both.

SUMMARY OF THE INVENTION

The present invention describes a novel method for effectively doublingof the sound producing efficiency in fixed and multiple harmonicfrequency types of plate and chamber and similar alarms, and itdemonstrates practical means for achieving this improvement.

In a plate and chamber alarm, a disk or plate is caused to vibrate at afrequency in the audible range, and most commonly in the range 200 Hz to4000 Hz. Vibration of the plate is typically produced by an electricalexcitation means, usually piezoelectric or electromagnetic in nature;less typically the vibration of the plate is produced by other meanssuch as air, mechanical, or hydraulic actuation. In many of thevariations of this class of alarm, the plate forms one side or wall ofthe acoustic chamber. Within the chamber, the vibrations of the plateare transferred to the air inside the chamber, and by means of familiaracoustic actions, the vibrating air in the chamber is caused to vibratein sympathy with the driving plate to form a resonant system. Suchresonance action greatly improves the ability of the vibrating plate totransfer its vibrations to create a strong, airborne acoustic signal. Incertain applications, an impedance-matching acoustic horn, and/or otherresonance enhancing or stabilization chambers and/or conduits also maybe used in conjunction with the chamber to further improve thecommunication of acoustic energy to the surrounding air. The design ofany specific vibrating plate and the acoustic chambers can be obtainedusing well-known design methods, and both classical acoustics and moderncomputational methods, e.g. finite element analysis (FEA) have beenapplied to these design problems. Typical devices are usually the resultof both analytical design and empirical developmental work, and thedevices described herein are the result of such combined methods.

It is a primary objective of the present invention to provide a methodfor significantly improving the acoustic efficiency of a plate andchamber type of audible alarm by utilizing the vibrations that exist onboth sides of the vibrating plate in a way that constructively combinesthe sounds so generated by both surfaces. Such constructive combinationof sound is achieved by the addition of an acoustic pathway or pathwayswhich first isolate sound generated in the resonating chambers from oneanother and then provide a differential acoustic delay such that theoriginally out of phase sounds combine together at the free air exit ofeach pathway in an additive fashion.

It is theoretically necessary for such additive combination to occurwhen the front side and backside conduits are caused to differ in theireffective acoustic lengths by one half wavelength of the sound at thegenerating frequency of the vibrating plate. A practical device ispossible, however, even if the actual effective path lengths differ onlyapproximately one half wavelength. Constructive addition will occurwhenever the pathways differ by more than one fourth of a wavelength andless than three fourths of a wavelength or any integral number ofwavelengths plus this range of variability. The amount of loss ofefficient combination is in fact quite small for even moderatevariations from the ideal half wavelength. Degradation of thecombinatorial effect varies as a cosine function, and for a device whosechange in path length is as much as thirty per cent longer or shorterthan the ideal length, the efficiency will be decreased from the idealdoubling by only about ten per cent. It is, therefore, relatively easyto achieve a very effective practical device for doubling or nearlydoubling of the sound producing efficiency. There are, as a consequenceof the available latitude in length of the delay means, many forms ofthe device of the present invention that can be produced effectivelyeven when manufacturing tolerances are significantly relaxed or when itis necessary because of space limitations to create delay conduits withnon-ideal lengths.

Furthermore, the notion of a delay conduit is a somewhat oversimplified,albeit accurate, way for accomplishing the required phase matching ofthe signals. In reality, the behavior of sound is quite complex,especially when considering its behavior within geometries whosedimensions are less than one wavelength. Because of the actualproperties of sound, it is possible to construct structures whosegeometrical effects on dispersion and diffraction also contribute toachieving the desired phase matching capability. Such alternativesolutions are frequently found by trial and error methods “at thebench.” More recently, however, finite element analysis (FEA) has becomea practical way to find such solutions. Present day FEA, performed on areasonably powerful personal computer, can provide an analyticaldescription of the behavior of sound that has simply not achievableusing more traditional “lumped parameter” methods for acoustic analysis.

While there is great permissible latitude in the lengths of the conduitsused to cause the constructive summation of the front and back generatedsounds, it is an extremely important consideration of the presentinvention that its fundamental airborne sound producing structure issuch that all of its parts are optimally chosen to support resonance atone or a small number of distinct frequencies. The so-designed alarm isspecifically not intended to operate over a large frequency band, aswould be the case, for instance, with audio speakers. The front and backchambers of the present invention, as well as the vibrating plate areall chosen with physical characteristics such that the sound is producedmost efficiently, i.e. at the system resonance(s).

Such efficiency thus permits any of the attributes loudness, size, orpower consumption to be optimized individually or collectively for aparticular application while permitting very large tolerances forvariability in manufactured devices.

This simple utilization of the heretofore “-unused” vibration surface onthe backside of the vibrating member then permits a variety ofapplication alternatives, particularly in the way that the soundphase-matching means are constructed. It is easily demonstrated thatwhen the construction of the device is such that the original soundchamber and vibrating plate are caused to remain unchanged, the additionof a secondary resonating chamber and a precise phase matching meanswill result in a 6 dB increase in the output sound pressure level (SPL),i.e. a doubling of the sound energy output.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification illustrate several aspects of the present invention, andtogether with the description, they serve to explain the principles ofthe invention. In the drawings:

FIGS. 1 a-1 h depict diagrammatically how a common plate and chamberalarm functions and how underlying principle of the present inventionextends the sound generating output power without requiring additionalinput power.

FIGS. 2 a-2 c depict a preferred embodiment of the present inventionwherein the secondary conduit is fashioned in the form of an axiallyfolded horn such as might exist, for example, in a general-purpose piezowhistle.

FIGS. 3 a-3 c depict a preferred embodiment of the present inventionwherein the secondary conduit is fashioned in the form of a radiallyextended horn whose terminal “bell” forms an annulus at right angles toits radial portion such as might exist, for example, in a smoke orcarbon monoxide detector.

FIGS. 4 a-4 c depict a preferred embodiment of the present inventionwherein the secondary conduit is fashioned in the form an additionalturn on a spiral horn such as might exist, for example, in anelectromagnetic automotive horn.

FIGS. 5 a-5 c depict a preferred embodiment of the present invention asa general purpose piezo whistle wherein the backside sound generation isformed by means of resonance chambers and conduits which are not mirrorsof the front side chambers and conduits and wherein the phase matchingof the front side and back side sounds is accomplished by elements whichare not necessarily exactly one-half wavelength delay conduits.

FIGS. 6 a-6 d depict details of practical means for constraining avibrating plate in the present invention.

FIGS. 7 a and 7 b depict useful configuration details for electricalcontact elements used with piezo-electric sound generating elements

FIG. 8 depicts a sound generating plate means for producing veryenergetic “click” sounds.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 depicts a progression ofillustrative structures that demonstrate the theoretical basis for theinvention. The particular aspects of the structures may be realized, ofcourse, by geometries of different, equivalent constructions, e.g. asquare instead of a circular boundary, or a magnetic instead of apiezoelectric drive, and the depictions of the FIGs must not be taken aslimiting but rather simply a convenience for illustrating the principlesin the invention.

Accordingly, FIG. 1 a depicts the simple plate and chamber system whichis further shown in cross section in FIG. 1 b, and which comprises atransducer element 10 in intimate contact with a support feature 20corresponding either to an edge or other vibration “node” of transducerelement 10, said support feature then being extended to form a chamberwall 24 of resonance chamber 30 where the vibration of the transducerelement 10 causes the springy air 35 (also more clearly depictedisolated alongside the structure as a hatched block) in the chamber 30to be sympathetically excited. An output port 40, typically consistingof a hole with or without interposed grillwork, is usually placed in thebottom of the chamber opposite the transducer element. Mass air 45inside the output port 40 is excited in concert with the springy air 35inside the chamber 40 to resonate in a similar manner to that entityknown as a “Helmholtz resonator” at a resonant frequency which depends,at constant temperature, pressure, and humidity, upon the mass andspring properties of the included air. Sound emanates from the outputport 40 to provide the alarm signal to the free air 90 of theenvironment. A variety of methods for electrically driving thetransducer element 10 are well known, and the electrical attachment istypically connected to the side of the transducer element that does notform a boundary of the resonance chamber. Sound generated by the meansdemonstrated in FIG. 1 a is most efficiently generated when the drivingfrequency of transducer element 10 is caused to equal the resonantfrequency of the sound system comprising the resonance chamber 30, itsspringy air 35, the output port 40, and the mass air 45. To a somewhatlesser degree, the sound of the system also is more efficientlygenerated when the driving frequency of the transducer element 10 isidentically caused to be equal to the resonance of the element 10 aswell as identically equal to the resonant frequency of the resonancechamber system.

In the structure depicted by FIG. 1 b, there can be a significant lossin energy transfer at the output port 40 to free-air 90 interfaces, andthere can as well be a susceptibility to interference with the air mass45 at the output port 40 which can disrupt the resonating system. FIG. 1c extends the sound generating concept toward providing a more stableresonance and more efficient coupling to the surrounding free air 90 bythe addition of transition chamber 50 and a transformer horn 70 as shownin FIG. 1 c. FIG. 1 d is further illustrative of a longitudinal crosssection of the depiction of FIG. 1 c, and in FIG. 1 d, transitionchamber 50 provides for a secondary springy air mass 55 to be capturedwithin secondary chamber walls and between output port 40 and secondaryoutput port which is shown as throat 72 of the transformer horn 70. Whena difference in acoustic impedance exists between the throat 72 and atthe mouth 76 of the transformer horn 70, the transitional nature of thetransformer horn serves to correct for such impedance mismatches betweenair in the horn and the ultimate impedance of the surrounding free air90. Such stabilizing and coupling means are readily and economicallyproducible using conventional production methods such as injectionmolding and stamping, and they may be incorporated into the simplerstructures of FIG. 1 a in many instances. The details shown in FIGS. 1 cand 1 d demonstrate the most complete description of such a stabilizedand well-coupled system.

While FIGS. 1 c and 1 d provide an optimal acoustic arrangement for aplate and chamber sound generating system operating at the resonantfrequency of its elements, FIGS. 1 e and 1 f show that there is nolimitation upon placing an identical back resonating chamber 30′ andback transition horn 70′ upon the backside of transducer element 10.Clearly the sound output from the back mouth 76′ of back transition horn30′ is equivalent to the sound output from the mouth 76 of horn 70 inits loudness and its frequency content. Just as clearly, the soundgenerated by the backside of transducer element 10 is also 180 degreesout of phase with sound generated by its front surface. With the twosounds being generated exactly out of phase, there will be cancellation,especially to the sides of a sound system so constructed.

To avoid such sound cancellation, FIGS. 1 g and 1 h depict the samesystem as FIGS. 1 e and 1 f, except that a delay means 80 has been addedto the structure. Delay means 80 is exactly one half wavelength of soundlong, and sound at the end of its path is exactly 180 degrees laggingthe sound at its input. In this way, the sound that emanates from theback mouth 76′ of back horn 70′ is exactly in phase with the sound thatemanates from the mouth 76 of horn 70. As a result, the sound generatedinto the surrounding air by the system of FIG. 1 d, is exactly twice asloud, or of 6 dB greater sound pressure level, than is the soundemanated by the similar half system originally presented in FIGS. 1 cand 1 d.

FIG. 2 a shows a perspective of the configuration of the presentinvention as a general purpose piezoelectric whistle 200 which has aconvenient single mounting nut 202 with serrations 204 for mounting thewhistle 200 in a panel with a single hole, the mounting nut 202 shown ina front mount position, and rear threads 206 are provided for analternative back mounting position

FIG. 2 b shows a cross-sectional view of general purpose piezoelectricwhistle 200 illustrative of the general concepts presented in FIG. 1,and in particular demonstrating delay means 80 as a “folded horn 270′for sound path 280′ in the present figure which is longer than soundpath 280 by one half wavelength of the resonant sound frequency of thedevice. This general purpose piezoelectric whistle 200 device iscomprised of a central piezoelectric transducer element 210 in intimatecontact with a stiffly compliant front support feature 220 shown as anO-ring in a groove feature 224, and a non-compliant back support feature220′ corresponding either to an edge support of transducer element 210.In order to maintain transducer element 210 in its side-to-side(centered) position in relationship to resonance chamber 230,edge-contacting pillars (not shown) are placed at a regular intervalsaround its periphery with a very small gap distance between theirperpendicular contacting line and the peripheral edge of centralpiezoelectric transducer element 210. Such centering pillars serve tokeep the plate from any significant drifting out of position withrespect to the chamber; for a circular chamber and plate, they serve tomaintain concentricity between the two elements.

The groove feature 224 of front support feature 220 is extended to forma chamber wall 226 of the front thin resonance chamber 230 where thevibration of the transducer element 210 causes the springy air 235 (alsomore clearly depicted isolated alongside the structure as a hatched airblock of FIG. 2 c) in the front thin resonance chamber 230 to besympathetically excited. A mirror of the front side structure of thedevice is also shown and is comprised of a back thin resonance chamber230′ having a back chamber wall formed by back support feature 220′which is shown in communication with the back surface of transducerelement 210 and containing back springy air 235′.

A front output port 240 is shown as a hole placed in the front wall ofthe front thin resonance chamber 230 opposite piezoelectric transducerelement 210. Mass air 245 inside the output port 240 is excited inconcert with the springy air 235 inside the chamber 240 to resonate at aresonant frequency which depends, at constant temperature, pressure, andhumidity, upon the mass and spring properties of the included air.Further, in mirror fashion to the front output port 240, a back outputport 240′ is shown in the back wall of the back thin resonance chamber230′ opposite piezoelectric transducer element 210. Mass air 245′ insidethe back output port 240′ is excited in concert with the springy air 235inside the chamber 240 to resonate at a back generated resonantfrequency identical to, but 180 degrees out of phase with, resonantfrequency of the front thin resonance chamber 230.

Contiguous with front output port 240 is transition chamber 250 whichencloses a secondary springy air mass 255 to be captured withinsecondary chamber walls and between output port 240 and secondary outputport 260. Secondary output port 260 is not a simple hole, but is ratherformed by a gap between the front sound cap 266 and the front structuralcup 264 and certain minor support legs 268 such as appear in section andpartial section in FIG. 2 c. Front structural cup 264 further includesthe surfaces that form the outer wall of the resonance chamber 230 andthe inner and sidewalls of transition chamber 250. A recess holdingmechanism 269 is depicted as a recess for its mating part. The air inthe output secondary port 260 is contiguous with air of the transformerhorn 270, which is very short in the present illustration. The horn isin contact with surrounding free air 90.

Similarly, contiguous with back output port 240′ is back transitionchamber 250′ which encloses a secondary springy air mass 255′ to becaptured within secondary chamber walls and between back output port240′ and back secondary output port 260′. Back secondary output port260′ is not a simple hole, but is rather formed by a gap between theback structural sound cap 266′ and the back horn housing cup 264′ andcertain minor support legs 268′ such as appear in section and partialsection in FIG. 2 c. Back horn housing cup 264′ further includessurfaces which form the outer wall of the resonance chamber 230′ and theinner and sidewalls of back transition chamber 250′. A companion barbholding mechanism 269′ is depicted as a barb for its mating feature,recess holding mechanism 269. The air in the back output secondary port260′ is contiguous with air in the throat 272′ of the back transformerhorn 270′, and the transformer horn includes additional length component280 which corresponds at least approximately to one-half wavelength ofthe sound frequency of the device at resonance. The mouth 276′ oftransformer horn is in contact with surrounding free air 90. The throat272′ and the mouth 276′ are connected by the transitional 278′ walls ofthe transformer horn 270′, which serve to correct for such impedancemismatches as exists between the output of the secondary resonancechamber and surrounding free air 90.

In the particular device of FIG. 2 b, the various cross sectionalconduits represent annular functional elements. Similarly the variousconnections between components such as the barb and recess holdingmechanism 269′ and 269; the cylinder 271 and star post 273 mechanism;and the stake 277 and socket 279 are present at intervals around thevarious annular placements in the assembly of general purposepiezoelectric whistle 200.

A variety of methods for electrically driving the transducer element 210are well known, and in the present device, the electrical contact withthe piezoelectric transducer element 210 is achieved by springcontactors 212 shown as cantilever extensions 214 of the springcontactors 212 proper which extend through contactor tunnels 218 passingfrom the chamber floor where it exists as a depression to the outer mostmargin of the structure of general purpose piezoelectric whistle 200 tobe attached to an electrical driving circuit and power supply (neitherof which is shown) external to the whistle structure.

FIG. 3 a shows a perspective of the configuration of the presentinvention as a smoke or carbon monoxide detector-housed-whistle 300which has an outer annulus 305 which may have typical wall mountingholes and battery receptacle (not shown) which are well-known and whichhave their design specified to a great extent by certain safetystandards such as UL 217 or UL 2034.

FIG. 3 b shows a cross-sectional view of detector-housed-whistle 300illustrative of the general concepts presented in FIG. 1, and inparticular demonstrating delay means 80 as a “radial turned horn” 370′for sound path 380′ in the present figure which is longer than soundpath 380 by one half wavelength of the resonant sound frequency of thedevice. This detector-housed-whistle 300 device is comprised of acentral piezoelectric transducer element 310 in intimate contact with astiffly compliant front support feature 320 shown as an O-ring in agroove feature 324, and a non-compliant back support feature 320′corresponding either to an edge support of transducer element 310. Inorder to maintain transducer element 310 in its side-to-side (centered)position in relationship to resonance chamber 330, edge-contactingpillars (not shown) are placed at a regular intervals around itsperiphery with a very small gap distance between their perpendicularcontacting line and the peripheral edge of central piezoelectrictransducer element 310.

The groove feature 324 of front support feature 320 is extended to forma chamber wall 326 of the front thin resonance chamber 330 where thevibration of the transducer element 310 causes the springy air 335 (alsomore clearly depicted isolated alongside the structure as a hatched airblock of FIG. 3 c) in the front thin resonance chamber 330 to besympathetically excited. A mirror of the front side structure of thedevice is also shown and is comprised of a back thin resonance chamber330′, having a back chamber wall formed by back support feature 320′ isshown in communication with the back surface of transducer element 310and containing back springy air 335′.

A front output port 340 is shown as a hole placed in the front wall ofthe front thin resonance chamber 330 opposite piezoelectric transducerelement 310. Mass air 345 inside the output port 340 is excited inconcert with the springy air 335 inside the chamber 340 to resonate at aresonant frequency which depends, at constant temperature, pressure, andhumidity, upon the mass and spring properties of the included air.Further in mirror fashion to the front output port 340, a back outputport 340′, is shown in the back wall of the back thin resonance chamber330′ opposite piezoelectric transducer element 310. Mass air 345′ insidethe back output port 340′ is excited in concert with the springy air335′ inside the chamber 340′ to resonate at a back generated resonantfrequency identical to, but 180 degrees out of phase with, resonantfrequency of the front thin resonance chamber 330.

Contiguous with front output port 340 is transition chamber 350 whichencloses a secondary springy air mass 355 to be captured withinsecondary chamber walls and between output port 340 and secondary outputport 360. Secondary output port 360 is not a simple hole, but is ratherformed by a gap between the front sound cap 364 and the front structuralcup 366 as appear in section and partial section in FIG. 3 c. Frontstructural cup 366 further includes the surfaces, which form the outerwall of the resonance chamber 330 and the inner and sidewalls oftransition chamber 350. A recess holding mechanism 369 is depicted as arecess for its mating part. The air in the front output secondary portis contiguous with air of the front transformer horn, both of which arevery short and shown simply combined as sound path 380 in the presentillustration. The horn is in contact with surrounding free air 90.

Similarly, contiguous with back output port 340′ is back transitionchamber 350′ which encloses a secondary springy air mass 355′ to becaptured within secondary chamber walls and between back output port340′ and back secondary output port 360′. Back secondary output port360′ is not a simple hole, but is rather formed by a gap between theback structural sound cap 364′ and the back horn housing cup 366′ andcertain minor support legs 368′ such as appear in section and partialsection in FIG. 3 c. Back horn housing cup 366′ further includessurfaces which form the outer wall of the resonance chamber 330′ and theinner and sidewalls of back transition chamber 350′. A companion barbholding mechanism 369′ is depicted as a barb for its mating feature,recess holding mechanism 369. The air in the back output secondary port360′ is contiguous with air in the throat 372′ of the back radial benttransformer horn 370′, and the transformer horn includes additionallength component 380 which corresponds at least approximately toone-half wavelength of the sound frequency of the device at resonance.The mouth 376′ of back radial bent transformer horn points in an axialdirection because of radial bend 374′, and air in the mouth 376′ is incontact with surrounding free air 90. The throat 372′ and the mouth 376′are connected by the transitional 378′ walls of the transformer horn370′, which serve to correct for such impedance mismatches as existsbetween the output of the secondary resonance chamber and surroundingfree air 90.

In the particular device of FIG. 3 b, the various cross sectionalconduits represent bent radial functional elements consistent with theshape of a smoke or carbon monoxide detector. There are variousconnections between components such as the barb and recess holdingmechanism 369′ and 369 which are particular to structures for suchapplications. Similarly, a battery 303, in a battery well 305 having abattery cover 307, which typically would include a lockout mechanism(not shown), also is presented for completeness of the disclosure. Theregion 309 inside of front structural cup 366 and back horn housing cup366′ provides room for a populated circuit board 315 and a detectionelement 317 also common to these applications. A variety of methods forelectrically driving the transducer element 310 are well known, and inthe present device, the means for making electrical contact between thepiezoelectric transducer element 310 and the driving electronics ofpopulated circuit board 315 is accomplished by either soldered wiring orspring contactors (neither of which is shown in FIGS. 3) whose fixedportions are external to the whistle structure. Ventilation means 319,consistent with smoke detector and/or CO detector function, are providedfor the free entry of air into the sensing area of the device.

FIG. 4 a shows a perspective of the configuration of the presentinvention as a vehicle audible alarm 400, which is typified by itspronounced horn structure. Audio alarms of this type may have a varietyof mounting features, which are well known to designers skilled in thisart, and the embodiment presented herein, while not showing any suchmountings, poses no new limitations on them.

FIG. 4 b shows an end view of vehicle audible alarm 400, and itidentifies the location of several sectioning lines 401, 403, and 405whose corresponding section views as FIGS. 4 c, 4 d, and 4 erespectively further clarify the internal structure of the embodiment.FIGS. 4 c and 4 d are illustrative of the concepts presented inparticular in delay means 80 in FIG. 1 h, with such delay beingaccomplished as the difference in length of a front spiral horn 470, anda back spiral horn 470′. Sound path delay 480 corresponds to a length ofone half wavelength of the resonant sound of vehicle audible alarm 400.

FIG. 4 e further shows, in cross section, the operational features ofvehicle audible alarm 400 comprising a central bi-stable platetransducer element 410 in intimate contact with a front support feature420, and back support feature 420′, which together comprise a fixed edgesupport of transducer element 410. Other accessories, not shown, includeappropriate gaskets and holding fasteners to seal and fix the assemblyfirmly on the periphery of central bi-stable plate transducer element410. A connecting rod 412 passing through a rod seal 414 connects thecentral bi-stable plate transducer element 410 to the electromagneticdrive motor 416.

A front thin resonance chamber 430 formed by the boundary the centralbi-stable plate transducer element 410 and front chamber wall 422contains springy air 435, which also is more clearly depicted as a crosshatched portion in FIG. 4 c. A mirror of the front side structure of thedevice is similarly comprised of a back thin resonance chamber 430′,formed by the boundary the central bi-stable plate transducer element410 and back chamber wall 422′ contains the back springy air 435′, whichalso is more clearly depicted as a cross hatched portion in FIG. 4 c.

A front output port 440 is shown as a hole placed in the front wall ofthe front thin resonance chamber 430 opposite central bi-stable platetransducer element 410. Mass air 445 inside the output port 440 is incommunication on one side with the springy air 435 inside the chamber430 and on its opposite side it is in contact with air in resonancechamber 450. Further, in mirror fashion to the front output port 440, aback output port 440′, is shown in the back wall of the back thinresonance chamber 430′ opposite central bi-stable plate transducerelement 410. Mass air 445′ inside the back output port 440′ is excitedin concert with the back springy air 435′ inside the chamber 430′ and onits opposite side it is in contact with air in back resonance chamber450.′

The throat air 455 in resonance chambers 450 is in direct communicationwith front horn air which extends within spiral horn 470 from its throat472 to its mouth 476. Similarly, in mirror fashion, the throat air 455′in resonance chamber 450′ is in direct communication with back horn air475′ which extends within back spiral horn 470′ from its throat 472′ toits mouth 476′. Air at the mouths of both horns is in contact withsurrounding free air 90.

The various shapes of the chamber and horn elements are selected suchthat their collective physical construction is consistent with thecontainment of such volumes and masses of contained air to cause it toresonate at a resonant frequency which depends, at constant temperature,pressure, and humidity, upon the mass and spring properties of theincluded air.

The details of the electromagnetic drive for the central bi-stable platetransducer element 410 are generally well known, and are represented inthe present device as an electromagnetic coil 404 surroundingmagnetopermeable slug 406 which is further connected to drive rod 412.Also affixed to drive rod 412 is switch actuator 408, which is inintermittent contact with switch 418. A leaf spring 413 and a setscrew415 further provide adjustment features to tuning capability to effectperiodic timing of electromagnetic drive.

FIG. 5 a shows a perspective of the configuration of the presentinvention as a practical, general purpose piezoelectric whistle 500which has a mounting base 502 with serrations two screw mounting holes504 for mounting the whistle 500 to a panel or similar basic structure.This embodiment serves to show that means for generating the backsidesound resonance and for obtaining a phase matching of the backside soundwith that of the front need not be an exactly one half wavelength longconduit.

FIG. 5 b shows a cross-sectional view of general purpose piezoelectricwhistle 500 illustrative of the general concepts presented in FIG. 1,and in particular demonstrating delay means 80 as a reflective open horn570′ for sound path 580′ in the present figure is diffetent from soundpath 580, but not necessarily longer by one half wavelength of theresonant sound frequency of the device. This general purposepiezoelectric whistle 500 device is comprised of a central piezoelectrictransducer element 510 in intimate contact with a stiffly compliantfront support feature 520 shown as an O-ring in a groove feature 524,and a non-compliant back support feature 520′ corresponding either to anedge support of transducer element 510. Small, line contact, centeringpillars (not shown) extending from non-compliant rear support feature520′ are placed at regular intervals around its periphery with a verysmall gap distance between their line contact and the peripheral edge ofcentral piezoelectric transducer element 510.

The groove feature 524 of front support feature 520 is extended to forma chamber wall 526 of the front resonance chamber 530 where thevibration of the transducer element 510 causes the springy air 535, alsomore clearly depicted isolated alongside the structure as a hatched airblock of FIG. 5 c, in the front thin resonance chamber 530 to besympathetically excited. A front output port 540 is shown as a holeplaced in the front wall of the front thin resonance chamber 530opposite piezoelectric transducer element 510. Mass air 545 inside theoutput port 540 is excited in concert with the springy air 535 insidethe chamber 540 to resonate at a resonant frequency which depends, atconstant temperature, pressure, and humidity, upon the mass and springproperties of the included air. The front chamber in its entiretyconstitutes a good approximation to a Helmholtz resonator, i.e. anacoustic device which, like a jug, creates a resonance with the air inthe neck acting like a mass and the air in the body of the bottle actinglike a spring.

On the other side of piezoelectric transducer element 510 a backsidestructure of the device is also shown and is comprised of a back thinresonance chamber 530′, having a back chamber wall formed by backsupport feature 520′, is shown in communication with the back surface oftransducer element 510 and containing back springy air 535′. (Backspringy air 535′ and other volumes and masses of air are more clearlydepicted in FIG. 5 c.) Further, a back output port 540′, is shown in theback wall of the back thin resonance chamber 530′ opposite piezoelectrictransducer element 510. Mass air 545′ inside the back output port 540′is excited in concert with the springy air 535′ inside the chamber 540′to resonate at a back generated resonant frequency identical to, but outof phase with, resonant frequency of the front resonance chamber 530.

Further comprising backside elements and contiguous with back outputport 540′ is back transition chamber 550′ which encloses a secondaryspringy air mass 555′ to be captured within secondary chamber walls andbetween back output port 540′ and back secondary output port 560′. Backsecondary output port 560′ is not a simple hole, but is rather formed bya gap between the back structural sound cap 566′ and the back hornhousing cup 564′ and certain minor support legs 568′ such as appear insection and partial section in FIG. 5 c. Back horn housing cap 566′further includes surfaces which form the outer wall of the resonancechamber 530′ and the inner and sidewalls of back transition chamber550′. A companion barb holding mechanism 569′ is depicted as a barb forits mating feature, recess holding mechanism 569. The air in the backoutput secondary port 560′ is contiguous with air in the throat 572′ ofthe back exit horn 570′. Sound entering region 574′ freely travels toreflection plane 575′ where it returns with forward velocity towardeffective rear mouth 576′ as a phase matched sound with that emanatingfrom front port 540 at a region 576 in free air 90.

In the particular device of FIG. 5 b, the various cross sectionalconduits represent annular functional elements. Similarly the variousconnections between components such as the barb and recess holdingmechanism 569′ and 569; the cylinder 571′ and star post 573′ mechanism;and the stake 577′ and socket 579′ are present at intervals around thevarious annular placements in the assembly of piezoelectric whistle 500.

A variety of methods for electrically driving the transducer element 510are well known, and in the present device, a circuit board 507 having anintegrated circuit 509 supported by passive components 511 is shownmounted in the base 502 of the device. The electrical contact with thepiezoelectric transducer element 510 is achieved by spring contactors512 shown as cantilever extensions 514 of the spring contactors 512proper which extend through contactor tunnels 518 passing from thechamber floor where it exists as a depression to the outer most marginof the structure of general purpose piezoelectric whistle 500 to beattached to an electrical driving circuit and power supply (neither ofwhich is shown) external to the whistle structure.

FIGS. 6 a to 6 d demonstrate means for achieving an effective “linecontact” method for positively holding a vibrating plate at a nodalpoint, which could also be an edge node. The various means, all shown incross section, serve to permit rotation while minimizing translation atthe support line for the vibrating element. The means shown in thevarious figures also provide for an effective sealing of the soundchamber.

FIG. 6 a shows a front chamber structure 602 having an annular groove tosupport a compliant O-ring 620 on one side of vibration plate 610, andit uses a back chamber structure 604′ which provides for an annular linecontact opposite the other side of vibration plate 610, the front andback support annular contact lines being exactly opposite one another.

FIG. 6 b shows a front chamber structure 602 having an annular groove tosupport a compliant O-ring 620 on one side of vibration plate 610, andit uses a similar back chamber structure 602′ which provides for anannular line contact also achieved by an O-ring 620 opposite the otherside of vibration plate 610, the front and back support annular contactlines being exactly opposite one another.

FIGS. 6 c and 6 d shows a front chamber structure 604 having anon-deformable annular line contact support on one side of vibrationplate 610, and a similar back chamber structure 604′ which provides foran annular line contact on the other side of vibration plate 610, thefront and back support annular contact lines being exactly opposite oneanother. FIG. 6 d additionally shows a hard mount 626 and a compliantmember, O-ring 622 placed between said hard mount 626 and backsidechamber structure 604′.

FIGS. 7 a and 7 b show details of contact means for bringing electricalpower to a piezoelectric sound generator element such as element 10.FIG. 7 a depicts a doubly cantilevered contact structure having a stem702, a first cantilever section 704 and a second cantilever structure706, which supports redundant contact points 708. FIG. 7 b depicts asingly cantilevered contact structure having a stem 712, a firstcantilever section 714 and a serpentine structure 716, which supportssingle arc-shaped contact point 718.

FIG. 8 shows details of a mechanical diaphragm useful in producing veryenergetic, short duration pulses or “clicks.” The device comprises asingle thin, elastic material diaphragm 810, which has a peripheralsupport edge 802 and a central driving pedestal 804. The plate isquasi-bi-stable, in that it will much more easily maintain a position attwo extremes as shown in FIG. 8 a and FIG. 8 b, with very little to nosustaining force. It requires substantial force, relatively speaking, tomaintain the plate at other positions between the tow extremes. An outersurround compliance feature 814 and an inner surround compliance feature816 support central flex region 818.

OPERATIONAL PRINCIPLES OF THE INVENTION

In operation, the fundamental aspects of the invention can be mostgenerally expressed in conjunction with the schematized pictorialdescriptions of FIG. 1. Referring now to the cross sectional view ofFIG. 1 b, a mechanical vibration is caused to occur in transducerelement 10 which in turn causes a compression and relaxation ofcontiguous springy air 35 inside resonance chamber 30. Springy air 35 isin continuous contact with mass air 45, and causes it to move as a moreor less block back and forth in output port 40. It will be appreciatedthat the springy air 35 is only approximately equivalent to a spring,and mass air 45 is only approximately equivalent to a block mass in whatis commonly referred to as a “lumped parameter” analysis of the system.The approximations are nonetheless adequate to describe the physics ofthe situation. The effect of the springy air 35 is to alternativelystore and release energy as it is first compressed and then expanded insympathy to the motion of transducer element 10. The effect of the massair 45 is to alternatively store and release energy as the air is causedto attain velocity in one direction, stop, and then reverse itself.Whenever such mass and spring elements exist, the system that they formwill attain a resonant frequency of operation, wherein the stored energyin the spring is exchanged for stored energy in the moving mass in avibratory manner. The particular frequency where such exchange occursoptimally is referred to as the “natural frequency” or resonance of thesystem. The surface of the mass air 45, which is opposite springy air35, is continuous with a spot of air in the environmental air 90. Atthis particular spot, energy is transferred from the mass air to theenvironmental air, and such energy, now referred to as acoustic energy,is free to propagate in all directions from the spot of its emanation.

The acoustic energy has itself both a pressure or potential energycomponent and a velocity or kinetic energy component as the energypropagates through the air. As the energy first emanates from the port40, it has a considerable mass characteristic, i.e., it is very muchlike the air inside the port, i.e. “hard”. The air surrounding the portis, in contrast, relatively “soft”, and there is a mismatch in such airat the interface. The so-called hardness or softness of the air aredegrees of an acoustic characteristic of the air commonly referred to asits acoustic impedance. Whenever the mismatch is severe, there is aninefficient transfer of energy between regions of harder or softerimpedance, and useful acoustic energy is lost. An acoustic transformer,in the form of a horn 70 is used to minimize such loss of energy byallowing the hard air of the port 40 to transition in a predeterminedsmooth expansive fashion to the environmental air 90.

As explained earlier, springy air 35 and mass air 45 are onlyapproximations of the behavior of the air contained in chamber 30 andport 40. In reality, the boundaries of the chamber and port can quitedramatically affect this assumption, and the system can fail to existwhenever certain compromises occur at the boundaries, particularly atthe boundary of the port 40 to the environmental air 90. This can exist,for instance, when there is velocity in the environmental air itself. Inorder to minimize such effects and other similar effects whichcompromise the mass-spring behavior, a secondary acoustic element suchas transition chamber 50 ase shown in FIG. 4 d is sometimes requiredintermediate to the port 40 and the outside air 90. It will beappreciated that such intermediate elements are quite specific to theparticular application in question.

The immediately preceding discussion describes how a simple plate andchamber alarm produces its sound to the environment, and it is clearthat such sound can be generated from the active front surface oftransducer element 10. However, as shown in FIG. 1 e, an equivalentstructure can be attached to the back surface of transducer element 10.The sounds emanating from the front horn 70 and the back horn 70′ whilebeing of identical frequency, will be exactly opposite in the timing ofthe pressure and relaxation parts of their energy transmission.Acoustically, a pressure or compression will be present at the hornmouth 76 exactly when a relaxation or rarefaction is present at hornmouth 76′.

The sound emanating from the respective horns can be made to add orsubtract to any degree by the application of a conduit or conduits whichprohibit the sounds generated from the respective surface to join in theenvironment 90. Such a delay conduit with a path length 80 is shown inFIG. 1 h. In this instance, the sound generated by the back surface oftransducer element 10 is caused to be delayed by a distance that isequal to one half of a wavelength of the resonant sound generated. Bykeeping the so generated sound from mixing in environmental air 90 withsound generated by the front surface of transducer element 10 until ithas traveled this extra distance, the sound emerging from its horn willnow be such that the sound energy at back horn mouth 76′ will exactlythe same frequency and phase as the sound emerging from the front hornmouth 76. The principle in practice will double the effective loudnessof the device over a device similarly constructed but employing only oneactive surface of the transducer element 10.

It is very important that the system so constructed be designed foroperation at one or at most a few related frequencies. The system willbe very efficient whenever the various path lengths are chosen in waysthat do not “spread” the effective resonance. In these devices, it isundesirable to seek any broadband response because such characterrapidly decreases the effectiveness and efficiency of the soundgeneration. That is, broad bandwidth can only be attained at thesacrifice of efficient energy transfer at a single very narrow bandsystem (or one that supports harmonics of such specific resonance.)

Certain operational aspects of the device are not peculiar to atwo-sided whistle, but are important to the production of a practicaldevice.

One so important operational consideration is in the mounting of thevibrating plate itself FIG. 6. In operation it is necessary for supportof the vibration plate to be provided which permit a tight acousticalseal and also prevent translation of the support of the vibration diskwhile allowing it to rotate through such contact. Practically, it isdifficult to achieve such holding, and a popular method has been to usea flexible adhesive such as a silicone RTV to hold the vibration elementin place and to seal it. A more predictable solution, as demonstrated inFIGS. 6 a, 6 b, and 6 d is the use of a compliant member to assure boththe acoustic seal and an effective line support. In FIG. 6 a, a hardline support on the backside of the disk is exactly opposite a compliantline contact formed by an O-ring 620. The O-ring 620, by permitting arelatively large amount of deformation compared to the manufacturingtolerances customary to the typical plastic structure, accommodates anymanufacturing variation while achieving the desired acoustic andstructural support requirements. FIG. 6 b accomplishes the same end, bututilizes two O-rings 620. FIG. 6 c demonstrates the use of two hard linecontacts, which in theory would provide the necessary acoustic seal andline support. However, in practice, it is difficult to assure that stackup tolerances will allow such perfect contact, and FIG. 6 d shows howthe addition of a compliant member, a spring, or as shown an O-ring 622,through its deformation, can be used to assure appropriate contact ofline edges of chambers 604 and 604′ by compliantly accommodating anystack up tolerance between support 626 and chamber structure 604′

Yet another important operational feature is the manner in whichelectrical contact is made for example to a vibrating piezoelectricsound-generating element 10 of FIG 1. In operational practice, suchelectrical contact can be accomplished by attaching leads directly tothe piezoelectric element, usually attaching such leads by wire bondingto the metallic structure or by soldering to the metallic structure andto a metalization that is bonded intimately with the piezoelectricceramic or other piezoelectric material used in the construction of thevibration plate. Commonly, however, mechanical contact is made to theplate using spring-loaded contactors. The contactors in common practicetypically have a single contact point, and the spring loading isaccomplished either by coil springs around cylindrical contacts or bysimple cantilever springs. An improvement on the general construction ofthe contacts is desirable which promotes redundant contact whileminimizing the overloading of the contact point itself, which canfracture the typical ceramic piezoelectric material common to theseapplications.

Referring now to FIG. 7 a, a doubly cantilevered spring contact ispresented wherein its stem 702 forms a quasi immovable base for thefirst cantilever portion 704 which in turn supports second cantileverportions 706 which provides a basis for redundant contact points 708. Inoperation, such double cantilever structure provides for very consistentcontact force over a relatively large range of deflection while assuringcontact at two points, which oppose each other in a seesaw fashion.

Referring now to FIG. 7 b, a singly cantilevered spring contact ispresented wherein its stem 712 forms a quasi immovable base for thecantilever portion 714 which in turn supports a basis for single contactpoints 708, which in this instance is a circular arc whose tangencyactually contacts the piezo element. In operation, Cantilever portion714 is further enhanced in its providing nearly constant force over awide range of deflection by means of serpentine 716.

Another important such operational consideration is that of a veryefficient sound generating diaphragm such as might find use in car hornsor other devices where substantial input energy is available and soundin the form of a regular series of highly energetic clicks is to beobtained. Such means is shown in FIG. 8, which in operation causes adisplacement of central driving pedestal 804 to store a significantamount of elastic energy in outer surround compliance feature 814, ininner surround compliance feature 816 and in central flex region 818.With increasing displacement the energy continues to be stored up to thepoint where the displacement causes the entire diaphragm to becomeunstable as an upward directed member, and the stored energy thenreleases very rapidly move the structure into its secondarily downwarddirected position. The mechanism is familiar as the click created by achild's toy “cricket.”

The foregoing description of preferred embodiments of the invention havebeen presented for purpose of illustration and description. It is notintended to be exhaustive nor to limit the invention to the precise formdisclosed. Obvious modifications or variations are possible in light ofthe above teachings. The embodiments were chosen and described in orderto best illustrate the principles of the invention and its practicalapplications to thereby enable one of ordinary skill in the art to bestutilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

1. A sound generating device comprising: a first acoustic chamber; asecond acoustic chamber; a plate interposed between the first and secondacoustic, the plate being capable of being vibrationally excited andoperative to generate sound in the first and second acoustic chamberssubstantially only at a resonant frequency common to both the first andsecond chambers and/or harmonics of the resonant frequency, the sound inthe first chamber having a phase difference from the sound in the secondchamber; and a phase adjustment circuit for adjusting the relativephases of sound generated in the first and second chambers so as to emitsound into the environmental air at approximately the same phase.
 2. Asound generating device as recited in claim 1 wherein the soundgenerated in the second acoustic chamber is 180 degrees out of phasewith the sound generated in the first acoustic chamber.
 3. A soundgenerating device as recited in claim 1 wherein the first and secondacoustic chambers are identical in their construction.
 4. A soundgenerating device as recited in claim 1 wherein the first and secondacoustic chambers are not identical in their construction.
 5. A soundgenerating device as recited in claim 1 wherein the phase delay circuitemits sound generated in the first and second chambers into theenvironmental air in generally the same direction.
 6. A sound generatingdevice as recited in claim 1 wherein the phase delay circuit emits soundgenerated in the first and second chambers into the environmental air atgenerally the same location.
 7. A sound generating device as recited inclaim 1 wherein the phase adjustment circuit adjusting includes a soundconduit of predetermined length and geometry.
 8. A sound generatingdevice as recited in 5, wherein the geometry of the sound conduit variesalong the length of the sound conduit.
 9. A sound generating device asrecited in claim 5 wherein the geometry of the sound conduit divergesalong the length of the conduit.
 10. A sound generating device asrecited in 6, wherein the geometry of the sound conduit varies along thelength of the sound conduit.
 11. A sound generating device as recited inclaim 6 wherein the geometry of the sound conduit diverges along thelength of the conduit
 12. A sound generating device as recited in claim1 wherein the phase adjustment circuit is in the shape of an axiallydisposed folded horn.
 13. A sound generating device as recited in claim1 wherein the phase adjustment circuit is in the shape of a spiral horn.14. A sound generating device as recited in claim 1 wherein the phaseadjustment circuit is in the shape of a conduit comprising a firstradial disposed portion followed serially by a second axial portion. 15.A sound generating device as recited in claim 1 wherein the phaseadjustment circuit is in the shape of an open conduit in the form of aplanar surface parallel to the sound wave emergent from the secondacoustic chamber and at distance from the port in the second acousticchamber such that the sound wave is reflected to travel approximately ahalf wavelength of sound to the point where it merges with the soundwave generated by the first resonance chamber.
 16. A sound generatingdevice comprising: a first acoustic chamber, a second acoustic chamber;a plate interposed between the first and second acoustic, the platebeing capable of being vibrationally excited and operative to generatesound in the first and second acoustic chambers substantially only at aresonant frequency common to both the first and second chambers and/orharmonics of the resonant frequency, the sound in the first chamberhaving a phase difference from the sound in the second chamber; at leastone resonance stabilization circuit for stabilizing the resonating soundgenerated in the first and second chambers so as to maintain a resonantair column over a range of variably occurring conditions due tomanufacturing, temperature, pressure and the like; and a phaseadjustment circuit for adjusting the relative phases of sound generatedin the first and second chambers so as to emit sound into theenvironmental air at approximately the same phase.